The research proposed here investigates the role that the prosody, or intonation, of mothers' speech to infants plays in the development of prelinguistic communication. When adults speak to infants, they use higher pitch and relatively smooth, simple and highly modulated intonation contours, when compared with normal adult speech. This research focuses on the developmental functions of this characteristic maternal speech style, known as "motherese". The first study examines the selective responsiveness of infants to motherese intonation, testing the hypothesis that infant cardiac orienting is greater to pitch contours typical of motherese than to pitch contours typical of normal adult speech. The second study tests the hypothesis that mothers consistently use specific intonation patterns to convey specific affective and pragmatic messages to prelinguistic infants. The proposed project has 3 general objectives: 1) to develop experimental methods for use in the study of language input, a research area that has traditionally relied on observational and descriptive methods; 2) to provide new convergent evidence on the role of maternal intonation in the development of communication and language; and 3) to use the proposed exploratory studies as a basis for developing an integrated future research program on the role of affective processes in the development of language.